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Magnetodynamics in Spin Valves and Magnetic Tunnel Junctions with Perpendicular and Tilted Anisotropies
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Material Physics, MF. (Applied Spintronics)ORCID iD: 0000-0001-9107-3309
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Spin-torque transfer (STT) effects have brought spintronics ever closer to practical electronic applications, such as MRAM and active broadband microwave spin-torque oscillator (STO), and have emerged as an increasingly attractive field of research in spin dynamics. Utilizing materials with perpendicular magnetic anisotropy (PMA) in such applications offers several great advantages such as low-current, low-field operation combined with high thermal stability. The exchange coupling that a PMA thin film exerts on an adjacent in-plane magnetic anisotropy (IMA) layer can tilt the IMA magnetization direction out of plane, thus creating a stack with an effective tilted magnetic anisotropy. The tilt angle can be engineered via both intrinsic material parameters, such as the PMA and the saturation magnetization, and extrinsic parameters, such as the layer thicknesses.

      STOs can be fabricated in one of a number of forms—as a nanocontact opening on a mesa from a deposited pseudospin-valve (PSV) structure, or as a nanopillar etching from magnetic tunneling junction (MTJ)—composed of highly reproducible PMA or predetermined tilted magnetic anisotropy layers.

      All-perpendicular CoFeB MTJ STOs showed high-frequency microwave generation with extremely high current tunability, all achieved at low applied biases. Spin-torque ferromagnetic resonance (ST-FMR) measurements and analysis revealed the bias dependence of spin-torque components, thus promise great potential for direct gate-voltage controlled STOs.

      In all-perpendicular PSV STOs, magnetic droplets were observed underneath the nanocontact area at a low drive current and low applied field. Furthermore, preliminary results for microwave auto-oscillation and droplet solitons were obtained from tilted-polarizer PSV STOs. These are promising and would be worth investigating in further studies of STT driven spin dynamics.

Abstract [sv]

Effekter av spinnvridmoment (STT) har fört spinntroniken allt närmare praktiska elektroniska tillämpningar, såsom MRAM och den spinntroniska mikrovågsoscillatorn (STO), och har blivit ett allt mer attraktivt forskningsområde inom spinndynamik. Användning av material med vinkelrät magnetisk anisotropi (PMA) i sådana tillämpningar erbjuder flera stora fördelar, såsom låg strömförbrukning och funktion vid låga fält i kombination med hög termisk stabilitet. Den utbyteskoppling (”exchange bias”) en PMA-tunnfilm utövar på ett intilliggande skikt med magnetisk anisotropi i planet (IMA) kan få IMA-magnetiseringsriktningen att vridas ut ur planet, vilket ger en materialstack med en effektivt sett lutande magnetisk anisotropi. Lutningsvinkeln kan manipuleras med både inre materialparametrar, såsom PMA och mättningsmagnetisering, och yttre parametrar, såsom skikttjocklekarna.

STO:er kan tillverkas som flera olika typer - som en nanokontaktsöppning på en s.k. mesa av en deponerad pseudospinnventilstruktur (PSV) eller som en nanotråd etsad ur en magnetisk tunnlingsövergång (MTJ) –och bestå av mycket reproducerbar PMA eller av skikt med på förhand bestämt lutning av dess magnetiska anisotropi.

MTJ-STO:er av CoFeB med helt vinkelrät anisotropi visar högfrekvent mikrovågsgenerering med extremt stort frekvensomfång hos strömstyrningen, detta vid låg biasering. Mätning och analys av spinnvridmoments-ferromagnetisk resonans (ST-FMR) avslöjade ett biasberoende hos spinnvridmomentskomponenter, vilket indikerar en stor potential för direkt gate-spänningsstyrda STO:er.

I helt vinkelräta PSV-STO:er observerades magnetiska droppar under nanokontaktområdet vid låg drivström och lågt pålagt fält. Dessutom erhölls preliminära resultat av mikrovågssjälvsvängning och av s.k. ”droplet solitons” hos PSV-STO:er med lutande polarisator. Dessa är lovande och skulle vara värda att undersökas i ytterligare studier av STT-driven spinndynamik.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 83 p.
Series
TRITA-ICT, 2016:21
Keyword [en]
perpendicular magnetic anisotropy, tilted-polarizer, spintronics, STT, MTJ, pseudospin-valve, STO, ST-FMR, magnonics, magnetic droplet, droplet nucleation, droplet annihilation
Keyword [sv]
vinkelrät magnetisk anisotropi, lutande polarisator, spinntronik, STT, MTJ, pseudospinnventil, STO, ST-FMR, magnonics, magnetisk droppe, nukleering av droppe, anihilering av droppe.
National Category
Nano Technology Condensed Matter Physics
Research subject
Physics
Identifiers
URN: urn:nbn:se:kth:diva-191176ISBN: 978-91-7729-072-8OAI: oai:DiVA.org:kth-191176DiVA: diva2:955796
Public defence
2016-09-30, Sal C, Electrum, Kungl Tekniska högskolan, Kistagången 16, Kista, 13:00 (English)
Opponent
Supervisors
Note

QC 20160829

Available from: 2016-08-29 Created: 2016-08-24 Last updated: 2016-08-29Bibliographically approved
List of papers
1. Depth-Dependent Magnetization Profiles of Hybrid Exchange Springs
Open this publication in new window or tab >>Depth-Dependent Magnetization Profiles of Hybrid Exchange Springs
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2014 (English)In: PHYS REV APPL, ISSN 2331-7019, Vol. 2, no 4, 044014- p.Article in journal (Refereed) Published
Abstract [en]

We report on the magnetization depth profile of a hybrid exchange-spring system in which a Co/Pd multilayer with perpendicular anisotropy is coupled to a CoFeB thin film with in-plane anisotropy. The competition between these two orthogonal anisotropies promotes a strong depth dependence of the magnetization orientation. The angle of the magnetization vector is sensitive both to the strength of the individual anisotropies and to the local exchange constant and is thus tunable by changing the thickness of the CoFeB layer and by substituting Ni for Pd in one layer of the Co/Pd stack. The resulting magnetic depth profiles are directly probed by element-specific x-ray magnetic circular dichroism of the Fe and Ni layers located at different average depths. The experimental results are corroborated by micromagnetic simulations.

National Category
Other Physics Topics
Identifiers
urn:nbn:se:kth:diva-156439 (URN)10.1103/PhysRevApplied.2.044014 (DOI)000344342600001 ()
Note

QC 20141203

Available from: 2014-12-03 Created: 2014-11-28 Last updated: 2016-08-26Bibliographically approved
2. Investigation of the Tunability of the Spin Configuration Inside Exchange Coupled Springs of Hard/Soft Magnets
Open this publication in new window or tab >>Investigation of the Tunability of the Spin Configuration Inside Exchange Coupled Springs of Hard/Soft Magnets
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2014 (English)In: IEEE transactions on magnetics, ISSN 0018-9464, E-ISSN 1941-0069, Vol. 50, no 6, 2004906- p.Article in journal (Refereed) Published
Abstract [en]

Magnetic multilayer (ML) structures comprising a perpendicular magnetic anisotropy (PMA) layer coupled to an in-plane magnetic anisotropy (IMA) layer are promising materials for zero/low field operating spin-torque oscillators and bit patterned recording media. The magnetization tilt angle can be easily tuned by varying the IMA layer thickness due to the competition between PMA and IMA layers. To explore the underlying magnetization reversal mechanism and to further understand the control of tilt angle and uniformity of the magnetization, the IMA (NiFe, Co, and CoFeB)/PMA (Co/Pd MLs) exchange spring systems are systematically studied. Experimental data obtained from magnetometry show good agreement with 1-D micromagnetic simulations, allowing us to design tunable exchange coupled spring as a function of IMA thickness.

Keyword
Competing magnetic anisotropy, exchange spring, tilted anisotropy materials, tunable magnetization
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:kth:diva-155813 (URN)10.1109/TMAG.2014.2299976 (DOI)000343033900005 ()2-s2.0-84957554152 (ScopusID)
Conference
1st International Symposium on Frontiers in Materials Science (IS FMS), NOV 17-19, 2013, Hanoi, Vietnam
Note

QC 20141113

Available from: 2014-11-13 Created: 2014-11-13 Last updated: 2016-08-26Bibliographically approved
3. Free- and reference-layer magnetization modes vs.~in-plane magnetic field in  a magnetic tunnel junction with perpendicular magnetic easy axis
Open this publication in new window or tab >>Free- and reference-layer magnetization modes vs.~in-plane magnetic field in  a magnetic tunnel junction with perpendicular magnetic easy axis
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2016 (English)In: Physical Review B Condensed Matter, ISSN 0163-1829, E-ISSN 1095-3795Article in journal (Other (popular science, discussion, etc.)) Accepted
Abstract [en]

We study the magnetodynamic modes of a magnetic tunnel junction with perpendicular magnetic easy axis (p-MTJ) in in-plane magnetic fields using device-level ferromagnetic resonance spectroscopy. We compare our experimental results to those of micromagnetic simulations of the entire p-MTJ. Using an iterative approach to determine the material parameters that best fit our experiment, we find excellent agreement between experiments and simulations in both the static magnetoresistance and magnetodynamics in the free and reference layers. From the micromagnetic simulations, we determine the spatial mode profiles, the localization of the modes and, as a consequence, their distribution in the frequency domain due to the inhomogeneous internal field distribution inside the p-MTJ under different applied field regimes. We also conclude that the excitation mechanism is a combination of the microwave voltage modulated perpendicular magnetic anisotropy, the microwave Oersted field, and the spin-transfer torque generated by the microwave current.

Place, publisher, year, edition, pages
American Physical Society, 2016
National Category
Condensed Matter Physics
Research subject
Materials Science and Engineering; Physics
Identifiers
urn:nbn:se:kth:diva-191175 (URN)
External cooperation:
Funder
Swedish Foundation for Strategic Research
Note

QC 20160829

Available from: 2016-08-24 Created: 2016-08-24 Last updated: 2016-08-29Bibliographically approved
4. Ultra-high frequency tunability in low-current and low-field spin-torque oscillators based on perpendicular magnetic tunnel junctions
Open this publication in new window or tab >>Ultra-high frequency tunability in low-current and low-field spin-torque oscillators based on perpendicular magnetic tunnel junctions
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(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

We demonstrate ultra-high frequency tunability of up to 4.4 GHz/mA, and low threshold currents of about -21 $\mu$A, in spin-torque oscillators based on CoFeB/MgO/CoFeB magnetic tunnel junctions, in which both free and fixed layers have perpendicular magnetic anisotropy (PMA). By using different thicknesses of the two CoFeB layers, their individual PMA strengths can be tailored to achieve significant relative misalignment of their respective magnetizations in moderate in-plane fields. We observe a broad maximum in both the device resistance and the generated microwave power around maximum misalignment. Maximum frequency tunability is observed at low-to-moderate fields and decrease rapidly after maximum misalignment.

Keyword
spin-torque oscillator, magnetic tunnel junction, magnetic perpendicular anisotropy
National Category
Materials Engineering
Research subject
Materials Science and Engineering; Physics
Identifiers
urn:nbn:se:kth:diva-191278 (URN)
External cooperation:
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20160829

Available from: 2016-08-26 Created: 2016-08-26 Last updated: 2016-08-29Bibliographically approved
5. Bias dependence of parallel spin torque in all-perpendicular magnetic tunnel junction nanodevices
Open this publication in new window or tab >>Bias dependence of parallel spin torque in all-perpendicular magnetic tunnel junction nanodevices
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(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

We performed spin-torque ferromagnetic resonance measurements on spin-torque oscillators based on CoFeB/MgO/CoFeB magnetic tunnel junctions (MTJs), with both free and fixed layers having perpendicular magnetic anisotropy, under the application of an in-plane field. A dc current between near-zero and -250 $\mu{A}$ was introduced to investigate the influence of spin-transfer torque effects on the asymmetric and symmetric components of the output voltage. Both components showed a minimum at a misalignment of about 30$^\circ$ between the magnetization of the two CoFeB layers, indicating a strong electric-field effect. We also found that the bias dependence of the spin-torque terms is in contrast to that of in-plane anisotropy CoFeB MTJs.

Keyword
PMA, ST-FMR, parallel spin torque, perpendicular spin torque, anisotropy field, electric-field effect
National Category
Materials Engineering Condensed Matter Physics
Research subject
Materials Science and Engineering; Physics
Identifiers
urn:nbn:se:kth:diva-191273 (URN)
External cooperation:
Funder
Swedish Foundation for Strategic Research Swedish Research CouncilKnut and Alice Wallenberg Foundation
Note

QC 20160829

Available from: 2016-08-26 Created: 2016-08-26 Last updated: 2016-08-29Bibliographically approved
6. Magnetic droplet solitons in all-perpendicular nano-contact spin torque oscillators
Open this publication in new window or tab >>Magnetic droplet solitons in all-perpendicular nano-contact spin torque oscillators
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(English)Manuscript (preprint) (Other (popular science, discussion, etc.))
Abstract [en]

Spin-torque-generated magnetic droplets are nontopological solitons that have both dynamic and static characteristics. Although   theoretical studies of these droplets originally dealt with  an all-perpendicular magnetic system, all experimental demonstrations have so far relied on orthogonal spin valve structures that require a rather strong magnetic field to nucleate the droplet. Here, for the first time, we show the nucleation and sustained operation of magnetic droplets under a low magnetic field using nanocontact spin-torque oscillators (NC-STO),  both the free Co/Ni and fixed Co/Pd multilayers of which have strong perpendicular magnetic anisotropy. Droplet nucleation is observed as a change in the NC-STO resistance and the appearance of significant broadband microwave signal generation below 1 GHz. We also observe another important phenomenon in which the nucleated magnetic droplet can transform into a skyrmionic nanobubble in the low-field regime. Both magnetic droplet solitons and skyrmionic nanobubbles have been studied in detail using micromagnetic simulation.

National Category
Condensed Matter Physics Materials Engineering
Research subject
Physics; Materials Science and Engineering
Identifiers
urn:nbn:se:kth:diva-191274 (URN)
External cooperation:
Note

QC 20160829

Available from: 2016-08-26 Created: 2016-08-26 Last updated: 2016-08-29Bibliographically approved

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